1. Field of the Invention
The present invention relates to a wiring for an electronic circuit comprising a laser contact portion which permits electrically connecting a plurality of conductor layers by utilizing a laser beam irradiation.
2. Description of the Related Art
In recent years, vigorous researches are being made in an attempt to develop new types of display devices replacing a cathode ray tube (CRT). Among the various new types of display devices, a liquid crystal display device, which is thin and can be operated with a small power consumption, is expected to find wide applications in the household electric appliances and OA equipments. Particularly hopeful is a liquid crystal display device utilizing an active matrix system exhibiting excellent display characteristics, in which a thin film transistor (TFT) is used as a switching element. As a matter of fact, attractive articles of the particular liquid crystal display device are being developed nowadays.
What should be noted, however, is that the liquid crystal display device utilizing the active matrix system tends to become larger in size and finer in mechanism, leading to an increase in the number of picture elements used and to an increase in the density of the picture elements. As a result, the pixel defect is likely to take place with an increased rate, leading to a marked reduction in the yield.
To overcome the difficulty described above, it is proposed to arrange an auxiliary TFT, which is not electrically connected to a pixel electrode of a pixel, in addition to a TFT connected to the pixel electrode. Where the TFT connected to the pixel electrode fails perform a normal operation, the auxiliary TFT is connected to the pixel electrode by utilizing a high energy beam such as a laser beam.
FIGS. 1A and 1B collectively show a laser contact portion in which a plurality of conductor layers superposed one upon the other with an insulating layer interposed therebetween are connected to each other by means of laser beam irradiation. Specifically, FIG. 1A is a plan view showing a plurality of conductor layers superposed one upon the other. In FIG. 1A, a reference numeral 1 denotes a first conductor layer, and 2, a second and conductor layer. A laser beam irradiation is applied to a region denoted by a reference numeral 6. Since a laser beam is applied from a region on the side of a substrate, the first conductor layer 1 is irradiated first with the laser beam in the practical operation. Incidentally, an insulating film is omitted from the drawing of FIG. 1A.
FIG. 1B is a cross sectional view along the line D.sub.1 -D.sub.2 shown in FIG. 1A. It is clearly seen that the first conductor layer 1, an insulating film 4, the second conductor layer 2 and a passivation film 5 are formed in this order on the surface of a substrate 3. In practice, a liquid crystal (not shown) is sealed in a region defined on the passivation film 5.
If a laser beam is applied in a direction E shown in FIG. 1B, the energy of the laser beam is absorbed first by the first conductor layer 1, with the result that the first conductor layer 1 is liquefied or gasified, leading to a volume expansion. It follows that the insulating film 4, the second conductor layer 2 and the passivation film 5 are broken to form a hole. What should be noted is that the liquid phase of the first conductor layer 1 is attached to wall of the hole formed as a result of the laser beam irradiation so as to permit the first conductor layer 1 to be fused to the second conductor layer 2. In this fashion, the first conductor layer 1 and the second conductor layer 2 are electrically connected to each other.
Where a laser beam is not applied, however, the laser contact portion naturally has a parasitic capacitance because the first and second conductor layers are superposed one upon the other with the insulating film interposed therebetween. The parasitic capacitance of the laser contact portion is so large as not to be negligible. On the other hand, the parasitic capacitance is eliminated in the case where the first and second conductor layers are connected to each other by the laser beam application. Naturally, the pixel in which the first and second conductor layers are connected to each other by the laser beam irradiation widely differs in the display characteristics from the pixel in which the first and second conductor layers are not connected to each other. This brings about nonuniform display characteristics so as to greatly impair the quality of the display device.
As described above, the conventional liquid crystal display device comprising a laser contact portion is defective in that the parasitic capacitance of the laser contact portion is so large as not to be negligible. As a result, the display characteristics of the liquid crystal display device are adversely affected seriously. What should also be noted is that the pixel in which an electrical connection is achieved in the laser contact portion by laser beam irradiation widely differs in the parasitic capacitance of the laser contact portion from the pixel in which the electrical connection is not achieved in the laser contact portion. As a result, the display characteristics of the pixels are rendered nonuniform, leading to a marked deterioration in the quality of the liquid crystal display device.